tests: Explicitly close stream in converter-stream

When the test has finished writing all the expanded content into the
socket, explicitly close the output stream, which should make the input
stream readable and non-blocking.

The code intended to do this before, but only as a side-effect of
dropping its last reference to `right`. If another reference was being
held to `right` somewhere else, it wouldn’t end up being closed, which
would lead to failures like
https://gitlab.gnome.org/GNOME/glib/-/jobs/1890000:
```
(/var/tmp/gitlab_runner/builds/Ff4WDDRj/0/GNOME/glib/_build/gio/tests/converter-stream:56570): GLib-GIO-DEBUG: 12:56:23.280: GSocketClient: Connection successful!
Bail out! GLib-GIO:ERROR:../gio/tests/converter-stream.c:1042:test_converter_pollable: assertion failed (error == NULL): Resource temporarily unavailable (g-io-error-quark, 27)
stderr:
```

This is a bit of a guess (I’m not sure it’ll fix the intermittent test
error, as I haven’t been able to reproduce that locally), but it’s worth
a try.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/tests/converter-stream.c

@@ -1017,6 +1017,7 @@ test_converter_pollable (void)
 	}
       else if (socket_out)
 	{
+	  g_output_stream_close (socket_out, NULL, NULL);
 	  g_object_unref (right);
 	  socket_out = NULL;
 	}